functionalization of the [closo-1-cb h anion for the ... · a structural element of new classes of...

214 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 214–225 ’ 2013 ’ Vol. 46, No. 2 Published on the Web 10/25/2012 www.pubs.acs.org/accounts10.1021/ar300081b & 2012 American Chemical Society

Functionalization of the [closo-1-CB9H10]�

Anion for the Construction of New Classesof Liquid Crystals

BRYAN RINGSTRAND† AND PIOTR KASZYNSKI*, †, ‡†Organic Materials Research Group, Department of Chemistry, Vanderbilt

University, Nashville, Tennessee 37235, United States, and ‡Faculty of Chemistry,University of Ł�od�z, Tamka 12, 91403 Ł�od�z, Poland

RECEIVED ON MARCH 16, 2012

CONS P EC TU S

T he [closo-1-CB9H10]� anion is a member of an extensive family of σ-aromatic closo-boranes that possess impressive stability

and functionalization characteristics. In contrast to its bigger, more extensively studied brother, the [closo-1-CB11H12]� anion,

convenient access to the [closo-1-CB9H10]� anion has only been recently established, and researchers have only begun to develop

and understand its fundamental chemistry.The geometrical and electronic properties of the [closo-1-CB9H10]

� anion make it an attractive structural element of novelclasses of either zwitterionic or ionic liquid crystals suitable for electro-optical and ion transport applications, respectively. Suchmaterials require a 1,10-difunctionalized [closo-1-CB9H10]

� anion that permits for the formation of molecules of elongated shape.The covalent attachment of an onium fragment or the use of a counterion compensates for the negative charge. This Accounthighlights the progressmade in the advancement and understanding of the fundamental chemistry of the [closo-1-CB9H10]

� anion.We also describe the development of 1,10-difunctionalized derivatives as key intermediates in the preparation of new classes ofliquid crystalline materials.

We obtained the first isomerically pure 1,10-difunctionalized derivative of the [closo-1-CB9H10]� anion, iodo acid [closo-1-

CB9H8-1-COOH-10-I]�, from decaborane through the Brellochs reaction. Functional group transformation of the C(1)-carboxyl

group led to a 1-amino derivative and, subsequently, to a synthetically valuable 1-dinitrogen derivative. The latter exhibitsreactivity typical for PhN2

þ and undergoes diazocoupling and Gomberg�Bachmann arylation reactions. The B(10)-iodineparticipated in Negishi alkylation and Buchwald�Hartwig amination reactions, leading to 10-hexyl and 10-amino carboxylic acids,respectively. We converted the 10-amino carboxylic acid to a 10-dinitrogen acid [closo-1-CB9H8-1-COOH-10-N2], which proved tobe synthetically valuable in the preparation of 10-pyridinium and 10-sulfonium zwitterionic acids and their liquid crystalline esters.We investigated several intermediates using structural, spectroscopic, and kinetic methods.

IntroductionThe discovery of polyhedral boron clusters was one of the

most exciting developments in inorganic chemistry of the

last century. These boron clusters are characterized by

three-center two-electron bonds, which were described by

William Lipscomb. Lipscomb's discovery, for which he was

awarded the Nobel Prize in 1976, revolutionized the way

chemists understood chemical bonding.1 As a result of this

unique bonding characteristic, certain boron clusters exhibit

σ-aromaticity and are exceptionally stable chemically, elec-

trochemically, and thermally.2 These properties opened up

possibilities for the application of boron clusters in a variety

of fields, such as treatment of nuclear waste and cancer,

catalysis, and materials science.3

Vol. 46, No. 2 ’ 2013 ’ 214–225 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 215

From [closo-1-CB9H10]� to Liquid Crystals Ringstrand and Kaszynski

The carboranes and the closo-borates have dominated

the literature due to their availability and relative ease

of functionalization.4 On the other hand, closo-monocarba-

borates, including the [closo-1-CB9H10]� (1) and [closo-1-

CB11H12]� (2) anions (Figure 1), have garnered less attention.

The negative charge in anions 1 and 2 is delocalized,

making the clusters non-nucleophilic and weakly coordinat-

ing anions.5 Critical for exploiting the properties of 1 and 2 is

the established chemistry of both the carboranes and bo-

rates. Deprotonation of the C�H vertex affords a carboranyl

anion capable of reacting with electrophiles.4 Conversely,

the B�H vertices are subject to electrophilic substitution,

leading to, for example, B-iodo derivatives, that have been

used in Pd-catalyzed coupling reactions.4

Of the closo-monocarbaborates 1 and 2, the [closo-1-

CB11H12]� anion (2) has been thoroughly investigated, and

as a result, its chemistry is more developed.6 On the other

hand, the [closo-1-CB9H10]� anion (1) has been mostly un-

explored due to poor accessibility. Anion 1 was discovered in

1971byW.H. Knoth as a product of thermal comproportiona-

tionof the [nido-7-CB10H13]�anion.7Othermethodsexpanded

onKnoth's work,8 but the preparation of 1,10-difunctionalized

species of anion 1 remained elusive. C-substituted derivatives

were rare before 2000,9,10 and electrophilic monohalogena-

tion was shown to occur solely at the 6-position.11

This situation changedwith development of the Brellochs

reaction, which permitted for the synthesis of a variety of

C(1) substituted derivatives of 1 and opened up new possi-

bilities for studying the {closo-1-CB9} framework.12,13 Ac-

cording to Brellochs, the reaction of decaborane, B10H14,

with aldehydes in alkaline solutionproduces either [arachno-

6-CB9H13-6-R]� (I) or [nido-6-CB9H11-6-R]

� (II) derivatives

(Scheme 1).13 These species undergo oxidative closure

of the {CB9} framework, giving [closo-2-CB9H9-2-R]� (III)

derivatives, and subsequent thermal rearrangement of III

provides the more thermodynamically stable [closo-1-

CB9H9-1-R]� (IV) isomer. The Brellochs reaction works well

with formaldehyde and electron deficient aldehydes.12�16

Despite an increase in the number of [closo-1-CB9H10]�

derivatives, 1,10-difunctionalized species were still practi-

cally unknown. Since electrophilic monosubstitution of

anion 1 occurs nearly exclusively at the 6-position,11 func-

tionalization of the 10-position posed a serious challenge.

However, it was demonstrated that halogenation of [closo-2-

CB9H9-2-Ph]� (III in Scheme 1) occurred regioselectively

at the 7-position, which after thermal rearrangement gave

a mixture of 1,10- and 1,6-disubstituted {closo-1-CB9} deri-

vatives.12 Although [closo-1-CB9H9-1-Ph-10-I]� was isolated

and its molecular structure established, a practical proce-

dure for the preparation of this or other 1,10-disubstituted

derivatives was not pursued.12 Such isomerically pure 1,10-

disubstituted derivatives could be used in the synthesis of

linear molecular systems17 and in our liquid crystals pro-

gram in particular.18

Liquid crystals are anisotropic fluids formed by aniso-

metric molecules typically containing both a rigid core and

flexible substituents, e.g. alkyl chains. One type of such

anisometric molecules is elongated, rodlike (calamitic), re-

presented in Figure 2. Typically, the molecular core contains

cyclic elements, such as benzene ring, cyclohexane, bicyclo-

[2.2.2]octane, or a heterocycle substituted in the antipodal

positions. The rigid core elementsmay be connected directly

or linked through functional groups such as esters, imines,

and azo or ethylene spacers.19 In such systems, liquid

crystallinity appears as a balance between ordering forces

FIGURE 1. [closo-1-CB9H10]� anion 1 and [closo-1-CB11H12]

� anion 2.The sphere represents a carbon atom, and all other vertices are BHgroups.

SCHEME 1. Brellochs Reaction and Preparation of [closo-1-CB9H9-1-R]� (IV)

FIGURE 2. Schematic representation of a rodlike molecule.

216 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 214–225 ’ 2013 ’ Vol. 46, No. 2


of the rigid cores and disordering effects of the alkyl

chains.

Rodlike molecules typically form nematic and smectic

liquid crystalline phases (Figure 3). The nematic phase is the

least organized phase, but it is the most widely employed in

the liquid crystal display (LCD) industry. Nematic liquid

crystals formed by polar anisometric molecules undergo

reorientation of molecules in an electric field, changing the

optical properties of the material. This electrooptical (EO)

effect is fundamental to virtually all LCD technologies. The

threshold voltage of the EO effect is approximately inversely

proportional to the magnitude of the molecular dipole.

Therefore, nematic liquid crystals and nematic phase-

compatible additives with large longitudinal or transverse

dipole moments are sought after for formulation of ne-

matic materials for display applications.

Ion pairs of charged anisometric molecules may also

form liquid crystalline phases. Such ionic liquid crystals

(ILC) typically form smectic phases and often exhibit high

ionic conductivity, making them attractive electrolytes for

solid-state batteries and dye-sensitized solar cells. The vast

majority of ILC are comprised of anorganic cation that drives

mesogenicity, such as pyridinium, with a small counterion

for charge compensation. However, anisometric anionswith

Liþ counterion are desired for battery applications.

These challenges in soft materials can be addressed by

using the [closo-1-CB9H10]� anion (1) as a structural element

of liquid crystals. Thus, disubstitution of cluster 1 in positions

1 and10with appropriate fragmentswill lead to anisometric

molecules, which is a prerequisite for the formation of liquid

crystalline phases. Compensation of the negative charge of

the anion with an onium substituent Qþ leads to zwitterions

V and VI having large longitudinal dipole moments. Such

molecules would be of substantial interest for electro-optical

applications. On the other hand, charge compensation with

a counterion may lead to anion-driven ILCs capable of

supporting cation transport (VII) (Figure 4).

Despite the potential of the [closo-1-CB9H10]� anion (1) as

a structural element of new classes of liquid crystals, under-

standing of its fundamental chemistry and availability of

1,10-difunctionalized derivatives were lacking. This review

will highlight the development of the fundamental chemis-

try of anion 1 and its application as a structural element of

new liquid crystalline materials.

TheFirst Isomerically Pure 1,10-Disubstituted {closo-1-

CB9} Cluster. We envisioned 1,10-disubstituted iodo acid

[closo-1-CB9H8-1-COOH-10-I]� (3) as a general precursor

to compounds V�VII (Figure 4). The B(10) iodine provides

a means to introduce alkyl, aryl, or amino functionality via

Pd-catalyzed coupling reactions. The C(1) carboxyl in3offers

another convenient synthetic handle, that can be converted

to esters or an amino group using classical organic synthetic

methods. The amino group at either the C(1) or B(10) posi-

tions can be transformed into a quinuclidine ring or synthe-

tically valuable dinitrogen derivatives.

The iodo acid 3 was prepared from [closo-2-CB9H9-2-

COOH] (4), obtained according to the Brellochs method.15

Iodination of 4 with N-iodosuccinimide (NIS) gave 7-iodo

derivative 5, which was thermolyzed, giving a mixture of

iodo acids 3 and 6. Isomerically pure iodo acid 3 was

obtained by crystallization (Scheme 2).20,21

Transformations of [closo-1-CB9H8-1-COOH-10-I]�.

With iodo acid 3 available in multigram quantities and

10% overall yield from B10H14,21 synthetic transformations

of the carboxyl group and iodine at positions C(1) and B(10),

respectively, to an amino group in the context of accessing

structures V�VII were investigated. Thus, the C(1) carboxyl

FIGURE 3. Schematics of nematic (N), smectic A (SmA), smectic C (SmC),and an isotropic liquid. The ovals represent themolecular rigid core, andthe squiggly lines alkyl chains.

FIGURE 4. Zwitterions V and VI and ion pair VII. Qþ represents anonium fragment such as ammonium, sulfonium, or pyridinium, or ametal cation (for VII only). R and R0 represent organic fragments.



was converted into an amino group using a modi-

fied Curtius rearrangement, via tert-butyl carbamate 7, pro-

viding [closo-1-CB9H8-1-NH2-10-I]� (8, Scheme 3).22 Initial

details of this transformation were developed for the parent

acid [closo-1-CB9H9-1-COOH]� (9), giving [closo-1-CB9H9-1-

NH2]� (10-C1).23

Transformations at the B(10) position began with the

Pd(0)-catalyzed Buchwald�Hartwig amination of iodo acid

3.24 Using excess lithium hexamethyldisilazane (LiHMDS) as

the ammonia equivalent and 2-(dicyclohexylphosphine-

)biphenyl as the phosphine ligand,21 the ratio of amino acid

11 to parent acid 9, a deiodination byproduct, was as high as

5:1 (Scheme 4).21,24 Using the same conditions, the 6-iodo

acid 6 was converted to 6-amino acid 12 (Scheme 5).25

Experiments demonstrated that the amination is substan-

tially more difficult for the {CB9} cluster than for aromatic

halides.

The iodine in 3 was also replaced with an alkyl group

via the standard Negishi coupling, providing, e.g., hexyl

derivative 13 (Scheme 6).22 Similarly, alkylation of iodo

amine 8 by typical Kumada coupling gave hexyl amine 14

as the sole product (Scheme 6).26 In both reactions,

a phosphonium zwitterion, resulting from replacement

of iodine with the phosphine ligand, was observed as a

byproduct.

N2þ Derivatives of the [closo-1-CB9H10]

� Cluster. With

access to both C(1) and B(10) amines, we focused on the

preparation of apical dinitrogen derivatives and their reac-

tions with nucleophiles. Diazotization of [closo-1-CB9H8-1-

NH3-10-R] (8, R = I; 10-C1, R = H) was performed in aqueous

AcOH, giving [closo-1-CB9H8-1-N2-10-R] (15, R = I;16-C1, R =H)

as a white precipitate (Scheme 7).22,23

Diazotization of amino acid 11 was surprisingly difficult,

and reaction under the conditions used for the C(1) amines 8

and 10-C1 led to full recovery of unreacted 11. Amino-closo-

borates are known to be unusually basic, which prompted us

to explore the diazotization under basic conditions. In fact,

diazotization of 11 and preparation of stable dinitrogen acid

[closo-1-CB9H8-1-COOH-10-N2] (17) was achieved with ex-

cess [NO]þ[PF6]� in MeCN containing pyridine (Scheme 8).24

The two types of dinitrogen derivatives, the C(1)�N2þ

(such as 16-C1) and B(10)�N2þ (such as 17), have different

thermal stabilities. While the C(1) derivatives decompose

SCHEME 2. Preparation of Iodo Acid 3

SCHEME 3. Preparation of Amine 8

SCHEME 4. Preparation of Amino Acid 11

SCHEME 5. 6-Amino Acids 12

SCHEME 6. Alkylation of Iodo Acid 3 and Iodo Amine 8

SCHEME 7. Preparation of Dinitrogen Derivative [closo-1-CB9H8-1-N2-10-R] (15 and 16-C1)



slowly in the solid state and more rapidly in solutions at

ambient temperature, the B(10) derivatives are far more

stable, and their thermal stability resembles that of dinitro-

gen derivatives of [closo-B10H10]2�.27 Temperature-depen-

dent NMR kinetic measurements revealed a first order

process with activation parameters ΔH‡ = 38.4 ( 0.8 kcal

mol�1 andΔS‡=44.5(2.5 calmol�1 K�1 inMeCN for [closo-

1-CB9H9-1-N2] (16-C1)23 and ΔH‡ = 33.9 ( 1.4 kcal mol�1

and ΔS‡ = 10 ( 3.5 cal mol�1 K�1 for methyl ester [closo-1-

CB9H8-1-COOMe-10-N2] (18) in PhCN.24 The rate-determin-

ing step in both reactions is the heterolytic cleavage of the

N2þ�boron cluster bond, leading to the formation of an

electrophilic ylide: carbonium 19-C1 from dinitrogen 16-C1

(Scheme 9), and boronium from dinitrogen 18. Subsequent

reaction of the ylide with a nucleophilic solvent leads to a

trapping product, such as 20.

MP2-level computational analysis of the parent dinitro-

gen derivatives 16-C1 and [closo-1-CB9H9-10-N2] (16-B10)

demonstrated that the B(10) isomer is more thermodynami-

cally stable by ΔH = 52.4 kcal/mol and has a higher

endotherm by 7.8 kcal mol�1 for the formation of the ylide

relative to that of the C(1) isomer16-C1.25 Consequently, the

boronium ylide 19-B10 is more thermodynamically stable

than the C(1) analogue 19-C1 by 44.6 kcal mol�1 and is

less electrophilic, according to the energies of the LUMO

(Figure 5).

The reactivity of [closo-1-CB9H9-1-N2] (16-C1) was inves-

tigated with three types of nucleophiles: sulfur compounds,

pyridine, andactivated arenes. The reactionswere conducted

tounderstand the formationof sulfoniumderivatives through

a masked mercaptan, the formation of 1-pyridinium deriva-

tives, and demonstration of a classical diazo coupling.23

Reaction of 16-C1 with N,N-dimethylthioformamide at

ambient temperature gave zwitterion 21, while reactions

with phenolate anion and aniline provided azophenol 22

andazoaniline23as sole products (Scheme10).23Alkylation

of protected mercaptan 21 with 1,5-dibromopentane gave

sulfonium derivative 24-C1 (Scheme 10).23

The reaction of 16-C1with pyridinewasmost interesting.

Thus, reaction in neat pyridine at ambient temperature gave

amixture of at least four products, instead of the anticipated

25a.23 The major component of the reaction mixture was

identified as 2-isomer 25b, while two minor more polar

components were identified as 25c and 25d (Figure 6). At

higher temperatures the composition of the mixture chan-

ged and the 1-isomer 25a constituted 17% of the mitxture.

Interestingly, a reaction of 16-C1 with a 4% solution of

pyridine in CH2Cl2 gave [closo-1-CB9H9-1-Cl]�, while a reac-

tion with a 4% solution of pyridine in benzene gave [closo-1-

CB9H9-1-Ph]� as the major product. In both reactions, iso-

mers 25a and 25b were formed in small quantities.

SCHEME 8. Preparation of Dinitrogen Acid [closo-1-CB9H8-1-COOH-10-N2] (17)

SCHEME 9. Thermolysis of [closo-1-CB9H9-1-N2] (16-C1) in MeCN

FIGURE 5. MP2/6-31G(d,p) derived contours and energies for theLUMO of ylides 19-C1 and 19-B10.

SCHEME10. Preparation of SulfoniumDerivative24-C1 andDiazocou-pling of 16-C1 with Phenolate and Aniline



The observed distribution of products suggests a radical

pathway, which is consistent with the Gomberg�Bachmann

radical arylation reaction. The process is initiated with single

electron transfer (SET) from pyridine, a soft nucleophile, to

16-C1 (Scheme 11). After loss of N2, the resulting radical

anion 26 undergoes addition to the heteroaromatic ring,

giving all four isomers 25, among which 25b dominates.

For a mechanism involving the carbonium ylide 19-C1 as

the intermediate, only one isomer, 25a, is expected, as

has been observed for dinitrogen derivatives of [closo-

B10H10]2�.27,28 This rationale is consistent with a tempera-

ture-dependent distribution of products: at low tempera-

ture the SET mechanism operates nearly exclusively,

yielding solely the C-isomers, while at high temperature

the heterolysis of the C(1)�N2 bond in 16-C1 is activated,

giving rise to measurable quantities of the N-isomer 25a

(Figure 6).

Additional support for the radical mechanism was pro-

vided by electrochemical analysis of 16-C1.23 The reduction

of 16-C1 is irreversible and more cathodic than that of

PhN2þ by only 0.38 V, rendering 16-C1 similarly susceptible

to SET from electron donors.

A similar mechanism involving radical ion 26 was postu-

lated for the reaction of 16-C1 with other soft nucleophiles,

such as Me2S and Me2NCHS, while with MeCN, a poor

electron donor, only the heterolytic mechanism operates

involving the ylide 19-C1.

Using the transformations developed for the parent com-

pound 16-C1, several intermediates for liquid crystalline

materials were prepared starting from iodo dinitrogen deri-

vative 15. This includes iodo derivatives 27 and 28, which

upon Negishi coupling with alkylzinc gave the 10-alkyl

derivatives 29 and 30[n], respectively (Scheme 12).22,26,29

Similar studies were conducted for acid [closo-1-CB9H8-1-

COOH-10-N2] (17), which was reacted with pyridine, sulfur

compounds, and alcohol. Again, the reactions were con-

ducted to prepare sulfoniumand 1-pyridinium zwitterions at

the B(10) position and to introduce a heteroatom link

between the alkyl group and the B(10) position.24 Thus,

reactions of dinitrogen acid 17 in neat pyridine, 4-heptylox-

ypyridine, N,N-dimethylthioformamide, and pentanol gave

pyridinium acids 31a and 31b, masked mercaptan 32, and

ionic pentyloxy acid 33, respectively, as the sole products

(Scheme13).24,30,31With CH2N2, the dinitrogen acid17 gave

methyl ester 18.24

The reaction of acid 17 with pyridine differs from that of

the C(1) dinitrogen derivative 16-C1. While the reaction of

16-C1 proceeded at ambient temperature and involved a

radical anion, the reaction of dinitrogen acid 17 with pyr-

idine required higher temperature and the formation of 31,

as the sole product, indicates domination by the heterolytic

pathway involving a boronium ylide intermediate. Thus, the

reactivity of 17 is similar to that of dinitrogen derivatives of

the [closo-B10H10]2� cluster with pyridine, which also show

enhanced thermal stability and reactivity implying involve-

ment of a boronium ylide.27 These characteristics of 17 are

consistent with a significantly cathodic and reversible reduc-

tion potential of anion 17� (E1/2 = �1.21 V vrs SCE), which is

apparently too low for effective SET from an electron donor to

trigger the radical process.24

The synthetic utility of the protected mercaptan 32 was

demonstrated by formation of sulfonium acids 34[n] as

shown in Scheme 14.21,24,32

Interestingly, the sulfonium acids 34[3] and 34[5] and

their esters exist as equilibrium mixtures of configurational

isomers trans and cis in a ratio of about 4:1 in solutions at

ambient temperature, while only the trans isomer is observed

for the series 28 and 30[n], in which the sulfonium fragment

is substituted at the C(1) carbon. Temperature-dependent

FIGURE 6. Product distributions in reaction of 16-C1 with neat pyridine. Asterisk denotes isolated yields.

SCHEME 11. Generation of Radical Anion 26



1H NMR studies demonstrated that the equilibrium between

the two isomers establishes quickly above ambient tempera-

ture, and yielded a difference in enthalpy, ΔH = 0.83 ( 0.04

kcal mol�1, and entropy, ΔS = 0.2 ( 0.1 cal mol�1 K�1,

between 35[5]c-cis and 35[5]c-trans (Figure 7).32

Computational results for model compounds 24-C1 and

24-B10 are consistent with experimental observations.32

The calculations demonstrated that 24-B10 has a low en-

thalpy of activation ΔH‡ = 24.2 kcal mol�1, which is con-

sistent with relatively fast isomerization observed for 35[5]c,

while ΔH‡ for 24-C1 is higher by about 7 kcal/mol. Calcula-

tions also demonstrated that the low cis/trans equilibrium

constant K = 2 for 24-B10 (experimental value for 35[5]c is

3.8) at 298 K is due to a relatively long B(10)�S bond (dS�B =

1.854 Å). In contrast, the C(1)�S bond in 24-C1 is shorter by

nearly 0.1 Å, and consequently, the axial conformer is less

favorable (K= 42). It is noteworthy that themolecular dipole

moment is substantial (Table 1) and oriented along the long

molecular axis, rendering both 24-C1 and 24-B10 attractive

structural elements for polar liquid crystals for display

applications.

We also investigated the generation and reactivity of the

dinitrogen acid [closo-1-CB9H8-1-COOH-6-N2] (36) starting

from 6-amino acid [closo-1-CB9H8-1-COOH-6-NH3] (12).25

Experiments using conditions for the analogous reaction of

the 10-amino acid 11 indicated that the dinitrogen carbox-

ylate anion 36� is unstable under the reaction conditions

and decomposes to a 6-boronium ylide, which is trapped

with the solvent. Diazotization of a mixture of amino acids

12 and 11 conducted in neat pyridine with [NO]þ[BF4]� at

�20 �C gave 6-pyridinium acid 37a as the sole product, and

trace amounts of dinitrogen acid 17 formed from 11

(Scheme 15).25

The reaction was extended to other heterocyclic bases.

Diazotization of a mixture of amino acids 12 and 11 in

SCHEME 12. Derivatives Prepared from [closo-1-CB9H8-1-N2-10-I] (15)

SCHEME 13. Transformations of Dinitrogen Acid 17

SCHEME 14. Preparation of Sulfonium Acid 34[n]

FIGURE 7. Epimerization of Ester 35[5]c



4-methoxypyridine gave 6-onium acid 37b in high yield. In

contrast, reactions in 2-picoline and quinoline gave incom-

plete conversion of 12 and low yields of the 6-onium

products 37c and 37d, and no diazotization of the 10-amino

acid 11 was observed. These results were attributed to the

interference of theMe group in NOþ transfer from 2-picoline

and lower basicity of quinoline as compared to pyridine.

The lack of diazotization of 10-amino acid 11 in less basic

quinoline and marginal reaction of 6-amino acid 12 are

consistent with the calculated lower acidity of the proto-

nated parent B(10)�amine 10-B10 than that of B(6)�amine

10-B6 (Table 2). The calculations for a set of isomeric

dinitrogen derivatives 16 are also consistent with the ob-

served significantly lower thermal stability of the 6-dinitro-

gen carboxylate 36�, relative to the 10-dinitrogen isomer

17�, and with the general order of stability of dinitrogen

derivatives of 1: B(10)�N2 > C(1)�N2 > B(6)�N2. Thus, the

MP2-level results predict a nearly 13 kcal/mol lower ΔG298

for dediazotization of 16-B6 and formation of 6-ylide 19-

B6, than for the 10-dinitrogen isomer 16-B10. Interestingly,

the thermal stability of the 2-dinitrogen derivative 16-B2

is predicted to be similar to that of the isolable stable

TABLE 1. Molecular Parameters for 24a

compd ΔH‡/(kcal mol�1) ΔG298 /(kcal mol�1) K298 dS‑Xb/Å μc/D

24-C1, X = C, Y = BH 31.2 2.2 1/42 1.754 14.424-B10, X = B, Y = CH 24.2 0.4 1/2 1.856 8.75aMP2/6-31G(d,p) level calculations with B3LYP/6-31G(d,p) thermodynamic corrections. bDistance between the S and the cage's C or B atoms in the axial conformer.cDipole moment of the equatorial conformer.

SCHEME 15. Diazotization Reactions of Amino Acids 11 and 12 in Heterocyclic Bases

TABLE 2. Selected Thermodynamic Parameters for Isomeric Derivatives of 1a

aReference 25.



1-dinitrogen derivative 16-C1. The reactivity of the resulting

ylides 19 varies, and judging from the energy of the LUMO,

the least electrophilic ylide is19-B6, while themost is19-C1.

Molecular Structures and Conformational Properties

of the [closo-1-CB9H10]� Derivatives. Molecular structures

of several derivatives, including dinitrogen derivatives 1533

and 17,24 pyridinium 25b,33 31a24 and 37a,25 sulfonium

acid 34[0],24 azo aniline 23,33 and quinuclidinium 38,26 were

established by single crystal XRD studies and compared toDFT

computational results. Two examples of XRD analysis with

indicatedpertinent interatomicdistancesare shown inFigure8.

Analysis of the {closo-1-CB9} cage geometry for a number

of structures indicates that the pyramidalization of the B(10)

apex is greater formore electronegative onium substituents.

The X�B(10) distance is smaller, and consequently, the

B(10)...C(1) separation is markedly shorter in dinitrogen acid

17 (dB(10) 3 3 3C(1) = 3.429 Å), with the most electronegative

substituent N2þ than in the sulfonium 34[0] (dB(10) 3 3 3C(1) =

3.478Å) and31apyridinium (dB(10) 3 3 3C(1) = 3.471Å) analogs.24

Conformational properties and, hence, the rigidity and

dynamic molecular shape impact mesogenic properties.

Both experimental and theoretical analyses indicate that

both the C(1)-carboxyl and B(10)-pyridine ring adopt a

staggered orientation with respect to the {closo-1-CB9} cage

in the conformational minimum. As a consequence, the

ideal dihedral angle between the planes of the two substit-

uents is 45�andall conformers of acid31are chiral (Figure9).

DFT results demonstrated that the thian ring at the B(10)

and C(1) positions prefers staggered conformations rela-

tive to the cage in which the sulfur lone pair eclipses the

B�B or C�B bond. Consequently, the acid 34[0] has a

Cs-symmetric conformational minimum, although in the

crystal structure the sulfonium ring is twisted by up to 30�away from the ideal staggered position. DFT analysis of the

B(10) alkyl and alkoxy derivatives 13 and 33 demonstrated

that the substituents are staggered and eclipsed relative to

the cage, respectively, resulting in chiral conformers in the

former and a Cs-symmetric molecule for acid 33.

Liquid Crystals Derived from the [closo-1-CB9H10]�

Cluster. The developed chemistry of the [closo-1-CB9H10]�

anion (1) permitted the preparation of the first examples of

two new classes of materials: polar zwitterionic (V and VI)

and anion-driven ionic liquid crystals (VII). The polar materi-

als V studied thus far, C(1)-sulfonium 30[n] and C(1)-quinu-

clidinium 38, do not form liquid crystalline phases

(Chart I).26,29 They exhibit high melting points and limited

solubility in other liquid crystalline materials; nevertheless,

they significantly increase the dielectric anisotropy, Δε, of

the host.26 For example, compound 38, with the calculated

longitudinal dipole moment component μll = 15.9 D, ex-

hibits a large value of Δε = 70 ( 1 extrapolated to infinite

dilution in a nematic host, and a relatively high virtual

clearing temperature, [TNI] = 139 ( 1.26

Polar materials VI, such as pyridinium esters 39 and

sulfonium esters 35[n], weremore promising thanmaterials

V (Chart II). Esters39have lowermelting points, have greater

FIGURE 8. Thermal ellipsoid diagramof [closo-1-CB9H8-1-COOH-10-N2](17) and [closo-1-CB9H8-1-COOH-10-C5H10S] (34[0]) drawn at 50%probability. Adapted with permission from ref 24. Copyright 2010American Chemical Society.

FIGURE 9. Extended Newman projection along the long molecularaxes of conformational minima for selected acids. The bars representthe substituents, and the filled circle is the connecting heteroatom orCH2 in 13.

CHART I. Examples of Polar Materials 30[n] and 38 (Type V)



compatibility with liquid crystalline hosts, and form nematic

phases above100 �C. Analysis of their dielectric properties indilute nematic solutions revealed extrapolated dielectric

anisotropy, Δε, typically around 40, but a record high value

of Δε = 113 was measured for cyanophenol ester 39e.31,32

Esters 39 permitted for the first time the effect of a di-

pole moment on the stability of the nematic phase to be

examined experimentally.31 This was accomplished by

comparing polar and nonpolar isosteric pairs of the [closo-

1-CB9H10]� anion and isoelectronic p-carborane, [closo-1,10-

C2B8H10]. Replacement of a polar B��Nþ fragment in 39

with a nonpolar C�C fragment provided compounds 40

with drastically different dipole moments (calculated Δμ ≈12 D for all pairs), but with essentially identical geometrical

and conformational properties (Figure 10).

The results for pairs of compounds such as 39c and

40c (Figure 10) demonstrated that a longitudinal dipole

moment's effect on phase properties strongly depends on

the nature of the substituent distant from the zwitterion

center.31 Thus, esters of cycloalkanols 39a/40a and 39b/

40b with the increased longitudinal dipole have higher

phase stability by about 55 K. A much smaller increase in

TNI of only 9 K is observed for the analogous pentylphenol

esters 39c/40c, and even moderate nematic phase destabi-

lization is found for the cyanophenol esters 39e/40e,

despite the same difference in the molecular dipole mo-

ment. These results suggest close intermolecular contact

and dielectric screening of the dipoles.

Esters 35[n] derived from sulfonium acids represent ex-

amples of a new class of shape-shifting nematic material,

where the molecule can exist in two different shapes (linear

or bent) due to facile isomerization at the sulfur center.29,32

This results in improved solubility in the nematic hosts and

lower transition temperatures TNI, when compared to esters

of pyridinium acid 31b.

Ion pairs with anisometric anions derived fromhexyl acid

13 or phenol 29 exhibit liquid crystalline phases and repre-

sent the first examples of anion-driven ILCs (type VII).

Typically, these materials possess the characteristic smec-

tic A phase (41�43, Chart III), commonly exhibited by ILC,

and occasionally a soft crystalline phase (41).22,34 Surpris-

ingly, compound 43 exhibits a nematic phase, which is

rarely observed in ILCs.34 Binary mixture studies demon-

strated that ILCs VII are miscible with nonionic LCs, which

is promising for formulation of materials for metal ion

conduction.

Interestingly, replacement of the connecting X = CH2

group in ionic liquid crystal 43 with an oxygen atom in 44

resulted in significant destabilization of the nematic and, to a

greaterextent, SmAphases, asestablished frombinarymixture

studies in the {closo-1-CB11} analogue of 43.35 DFT modeling

ofbothanions suggests that thedestabilization results fromthe

CHART II. Examples of Polar Materials (Type VI)

FIGURE 10. Polar and nonpolar isosteric derivatives 39c and 40c.

CHART III. Examples of Ionic Liquid Crystal (Type VII)a

aEach anion is accompanied by 1-butyl-4-heptyloxypyridinium.



difference in charge distribution rather than conformational

changes.

Further examples of polar and ionic liquid crystals derived

from the [closo-1-CB9H10]� anion are being investigated in

our laboratory with the focus on greater compatibility with

nematic materials and ion pairs with the Liþ cation.

SummaryThis Account presented a snapshot of the rapid develop-

ment of the fundamental chemistry of the [closo-1-CB9H10]�

anion (1). Three key advancements in the fundamental

chemistry of 1 were highlighted: (i) the multigram scale

preparation of the isomerically pure 1,10-disubstituted deri-

vative, iodo acid 3, (ii) functional group transformations at

the C(1) position, and (iii) functional group transformations

at theB(10) position. Transformations of iodo acid3have led

to other important intermediates, such as dinitrogen deriva-

tives 15 and 17. These developments have opened up

exciting possibilities for application of anion 1 as a structural

element in a variety of molecular and polymeric materials,

including the new classes of polar (V andVI) and ionic liquid

crystals (VII).

BIOGRAPHICAL INFORMATION

BryanRingstrand receivedhis Ph.D. degree in2011at VanderbiltUniversity, where he worked with Prof. Piotr Kaszynski. He iscurrently a postdoctoral appointee at Argonne National Laboratory.

Piotr Kaszynski received his M.S. degree from Warsaw Poly-technic (1985) and subsequently his Ph.D. fromUniversity of Texas(1991). After a postdoctoral appointment at Caltech, he joinedthe faculty of Vanderbilt University (1993) and University ofŁ�od�z (2010). His research interests are in the areas of molecularmaterials, liquid crystals, boron clusters, stable radicals, and strainedpolycyclic systems.

FOOTNOTES

*To whom correspondence should be addressed: e-mail, [email protected] authors declare no competing financial interest.Dedicated to the memory of Professor Lee J. Todd (1936�2011).This project was supported by NSF grants (DMR-0606317 and DMR-0907542).

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functionalization of the [closo-1-cb h anion for the ... · a structural element of new classes of...

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